Erase saturation refers to the inability to erase a floating gate memory cell by removing charge from the floating gate to the Si channel through the tunnel oxide. This effect occurs because the parasitic current injected from the control gate towards the floating gate through the interpoly dielectric. The same problem arises in charge trapping non-volatile memory cells in which charge is stored in a charge trapping gate and the upper dielectric is called the blocking dielectric.
A way to avoid erase saturation consists in using high-k dielectrics with high-workfunction metal gates. However, metal gates are difficult to integrate in a conventional process flow. In addition, they show a tendency to change the effective workfunction towards midgap, when deposited on some dielectric materials, likely consequence of the inherent thermal steps following their deposition. Use of a p-type poly-silicon control gate, as is a trend nowadays, may be compromised by this so-called Fermi level pinning (FLP) effect. Furthermore, using a p-type poly-Si may require application of a higher erase voltage, in order to compensate for possible polysilicon depletion effect.